The petroleum industry relies heavily on the operation of drilling into the earth, both on land and offshore, in the exploration for and production of petroleum products. Over the years, the more readily found and accessible petroleum reservoirs have of course been discovered and depleted first. As a result, the exploration and production operations must necessarily concentrate to a greater degree on less accessible and less readily discoverable reserves. In order to reach these locations, the depths of drilling have increased, the locations at which drilling takes place have become increasingly difficult and less accessible, and the drilling operations have necessarily become more complex. Accordingly, drilling operations in the search for and production of petroleum products have become more expensive, with this trend likely to continue in the future. Because of this increasing cost, the accuracy and efficiency of the drilling operation is becoming even more important.
The success and efficiency of the drilling operation depends to a large degree on the quantity and quality of information that the drilling operator has about the sub-surface structure into which the drilling is taking place, and also about parameters concerning the operation of the drill bit as it proceeds into the earth. Many techniques for acquisition and communication of such information have been tried and used in the industry. Recent work has been done, as will be discussed hereinbelow, in acquiring information from the acoustical vibrations in the drill string itself during drilling. In such an application, the drill string serves not only to power and guide the drilling, but also as a communication medium for such acoustic signals. These signals are inherently generated during the drilling operation and communicated via the drill string to detectors. Analysis of the signals provides information about the drilling parameters and the drilling operation itself, and also about the geology into which the drilling is taking place.
An example of a system and method using acoustic vibrations transmitted along the drill string itself to communicate various drilling parameters is described in U.S. Pat. No. 4,715,451, issued Dec. 29, 1987, assigned to Atlantic Richfield Company, said U.S. Pat. incorporated herein by this reference. This system measures the motion of, and the strains on, a drillstem in various directions, by way of monitoring such indications as axial, torsional and lateral vibrations, and deflections of the drillstring. The strain generated on the drill string during drilling is indicative of such factors as the impact and rotation of the drill bit, its interaction with the formation into which the drilling is taking place, and the interaction with portions of the drill string above the bit with the surrounding formation. In this system, measurements are made by way of detectors, such as accelerometers and strain gages, which are located in a sub near the top of the drill string and which generate electrical signals corresponding to the vibration and motion detected thereby. Analysis of the electrical signals provides real-time information on parameters such as drillstem vibration and deflection, the location of interaction between the casing and the drillstem, the speed of and load on the drill bit, and other drill bit operating characteristics. Such real-time operating information is quite useful in efficiently and accurately performing the drilling operation.
As disclosed in said U.S. Pat. No. 4,715,451 at column 5, lines 59 through 68, in Rector III, et al., "Extending VSP to 3-D and MWD: Using the drill bit as downhole seismic source", Oil and Gas Journal, (Jun. 19, 1989), pp. 55-58, and in Rector, Marion and Widrow, "Use of Drill-Bit Energy as a Downhole Seismic Source", 58th International Meeting of SEG, paper DEV 2.7, pp. 161-164, analysis of the vibrations communicated along the drill string during drilling is useful in the seismic prospecting area, where the vibrations generated by the drill bit into the earth are the seismic source signals. Relative to the TOMEX.RTM. (Trademark of Western Atlas International Inc.) system disclosed by Rector III et al., seismic detectors such as geophones or hydrophones detect the reflections of these vibrations near the surface at a location distant from the drilling operation. Detection of the vibrations at the wellhead, as communicated by the drill string, can provide a signature of the source vibrations. Conventional time-domain cross-correlation of the vibrations detected by the geophones or hydrophones with the source vibrations communicated through the drill string provides data concerning the location of sub-surface strata and interfaces.
Another system which utilizes the drill string as a medium for the transmission of data is referred to as stress wave telemetry. Stress wave telemetry systems are disclosed in copending U.S. patent applications Ser. No. 188,231 filed Apr. 12, 1988, now U.S. Pat. No. 4,992,997, issued Feb. 12, 1991, Ser. No. 554,030 filed Jul. 16, 1990, and in Ser. No. 554,022 filed Jul. 16, 1990, all applications also assigned to Atlantic Richfield Company, and incorporated herein by this reference. This system includes transmitters, such as solenoids eccentric motors, and piezoelectric transducers, which intentionally vibrate the drill string in a manner corresponding to the desired data. This data may include information concerning drilling parameters, such as in the above-referenced U.S. Pat. No. 4,715,451. In the case of stress wave telemetry, however, the information is not extrapolated from analyzing the naturally occurring vibrations, but vibrations are generated which are in addition to the naturally occurring vibrations, these generated vibrations corresponding to the drilling parameter and other information transmitted along the drill string.
It has been discovered, however, that vibrations, whether from the drill bit itself or intentionally generated by transmitters, are not communicated through the drill string in an ideal manner, due to the non-ideal response of the drill string to such vibrations. As described in Drumheller, "Acoustical Properties of Drillstrings", J. Acoustic Society of America, 85(3) (March, 1989), pp. 1048-1064, conventional drill strings, which consist of a number of lengths of drill pipe joined by pipe joints, inherently have frequency domain stopbands which attenuate acoustical signals at the stopband frequencies. This frequency-dependent attenuation can severely distort some signals. While simple deconvolution of the reflective effects of the ends of the drill string and the bottomhole assembly has been done, such deconvolution has accounted only for effects dependent upon the total length of the drill string and the construction of the bottomhole assemble, and has not accounted for the frequency dependent transmission of the drill string due to such factors as the tool joints between sections of the drill string.
Furthermore, it has been discovered that other factors also distort the vibrations communicated along a drill string from downhole to the surface. Such factors include noise generated by the drilling fluid, or mud, which is conventionally pumped through the drill string at relatively high pressures. This high pressure flow of fluid causes significant vibrations in the drill string. Other apparatus in the drilling operation, such as bearings in the swivels at the top of the drill string, the rattling of chains which turn the kelly bushing, or the motor in a top drive drilling arrangement, and the slap of the casing against the drill string or well bore, also generate significant acoustical vibrations which are received by and transmitted along the drill string. These vibrations are superimposed upon the vibrations generated by the drill bit, and will accordingly be detected at the top of the drill string by such detectors as are attempting to detect the vibrations which are induced by the drill bit.
Considering the vibrations generated by the drill bit, or alternatively the vibrations generated by a transmitter in the stress wave telemetry case, as "signal", and considering the other vibrations caused by drilling mud flow and the mechanical sources discussed in the prior paragraph as "noise", it has been found that the amplitude of the noise can be 1000 times greater than the signal amplitude. Noise at this level not only clouds the analysis of the information, but indeed drowns out the information itself. This is true in the contexts of determining real-time drilling parameters, producing a source signature from the drill bit for seismic prospecting, and in the case of stress wave telemetry.
The presence of such noise and distortion has been observed in field tests of the TOMEX.RTM. system described hereinabove, by J. P. DiSiena et al., "VSP While Drilling: Evaluation of TOMEX", Exploration Technology Report (Atlantic Richfield Company, Fall 1989), pp. 13-20, incorporated herein by this reference. While recovery of the drill-bit signal in the seismic prospecting context could be done in drilling operations using tricone bits and rotary drive, the signal-to-noise ratio for operations using PDC (Polycrystalline Diamond Compact) bits or a downhole mud motor was so low that no seismic source record could be detected in the drill string vibrations. The signal-to-noise ratio of the drill-bit energy as transmitted along the drill string must therefore be improved in order for such seismic prospecting analysis to be feasible during drilling, when using such important drilling equipment.
It is therefore an object of this invention to provide a method of reducing the effects of noise on information communicated along a pipe structure such as a drill string.
It is a further object of this invention to provide such a method which also includes the deconvolution of the pipe string response in accomplishing the noise reduction.
It is a further object of this invention to provide such a method together with an improved deconvolution, taking into account the passbands and deadbands of a jointed pipe structure.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to this specification, together with the drawings.